This invention relates to torque transducers for measuring the magnitude of torque in shafts, in particular rotating shafts such as found in electric power steering systems in vehicle applications.
Electric power steering systems conventionally incorporate an input shaft element, connected via an intermediate shaft and Hookes joint arrangement to the steering wheel. The input shaft therefore needs to rotate through an angle typically one to two revolutions either side of the on-centre steering position. The input shaft is at least partially surrounded by the fixed housing of the steering gear. It is a requirement of the electric power steering servo system to accurately measure the continuously varying torque in this rotating shaft. Conventionally torque applied to the shaft causes it to is angularly deflect, such deflection causing one part of the shaft to angularly displace with respect to another part, and this displacement is sensed to provide a measurement of this torque.
The sensing means needs to allow for rotation of the shaft within the housing, usually employing non-contact or mechanical signal transmission means. Non-contact means include optical aperture based devices and magnetic devices such as magnetostrictive or variable reluctance couplings. Mechanical means include slidably connected potentiometers and other indicating devices.
To improve the accuracy of such sensing means a torsionally compliant coupling in the form of a torsion bar is used to connect the two input members at either end of the shaft When torque is applied between the two input members the torsion bar deflects causing an increased angular displacement, which allows the use of less sensitive, or less accurate sensing means.
The torsion bar may be in the form of a separate element as in the case of a conventional rotary hydraulic power steering valve. Alternatively, in the case of some proposed electric power steering systems, the torsion bar may in fact be integral with the shaft member and be a relatively torsionally compliant (ie. less torsionally stiff) portion of the shaft member which couples substantially rigid torque input members at each end of the shaft member. The shaft member in these latter systems can be readily machined as a single steel component, and the only requirement is that the angular deflection of the relatively torsionally compliant coupling portion, connecting the two substantially rigid torque input member portions, has sufficiently low torsional stiffness that the sensing system is able to accurately measure its angular deflection.
Generally, the use of a torsion bar requires the use of a failsafe mechanism, being a torque limiting device to prevent failure of the torsion bar when unavoidable torque overload conditions occur.
Such torque limiting devices are well known in the art of vehicle steering, and will therefore not be described in this specification.
The prior art, which is most closely related to that of the present invention, is described in U.S. Pat. No 5,369,583 and International Patent Application PCT/GB95/02017 which show sensors employing optical disc apertures for measuring torque.
The essence of the present invention resides in the provision of grating elements comprising surfaces composed of alternating regions of high and low reflectivity connected by a torsionally compliant coupling. These surfaces are illuminated by a source of electromagnetic radiation (EMR), typically UV, visible or IR light, which generates patterns on one or more arrays of detectors sensitive to the EMR. Arrays include CCD devices, VLSI vision chips, one and 2 dimensional photodetector arrays and lateral effect photodiodes (commonly referred to as PSD""s or position sensitive devices). The disposition of the patterns is a function of torque applied to the shaft, and the output of the one or more arrays can be processed to produce a measure of the torque applied to the shaft. It is distinguished from other reflective torque transducers by use of an reflective imaging approach which does not rely on Moire fringes, speckle patterns or other diffraction gratings. As it uses photo detector arrays, EMR reflected from the gratings provide an instantaneous image which allows a much faster and more complete means of interpreting the information than is possible with individual photo-detectors. In the latter case it is necessary to count successive changes of EMR intensity incident on the photo-detector, which is slower and more prone to error.
Another reflective torque transducer that uses arrays is described in U.S. Pat. No. 5,490,430. This relies on a change in diffraction angle of two or more diffraction gratings that are torsionally strained by the application of torque. This device is prone to error due to misalignment and bending load and requires a collimated and monochromatic source of EMR. The regions of high and low reflectivity can be arranged axially or radially about the axis of rotation of the shaft, and are of such a nature that allows a continuous output of the arrays at any instant in time regardless of the angular position of the shaft, as the limited array dimensions may not allow the complete circumference or radial face to be viewed by the arrays. The advantages of such a construction over that disclosed in U.S. Pat. No. 5,369,583 and International Application Number PCT/GB95/02017 may arise as one or more of the following:
Firstly, the use of reflective grating elements allows simpler and more compact construction by the use of a cylindrical grating element arrangement, which is not readily achievable using disc apertures as shown in the prior art without requiring a significantly increased diameter. It also allows the EMR source(s) and array(s) to be packaged in the same assembly with further savings in space and cost. Secondly, it allows for easy assembly and disassembly of the transducer, as the grating elements can be removed from one end of the transducer in an axial direction without disturbing the EMR source(s) or array(s).
Thirdly, another advantage with the use of reflective grating elements is that the EMR is reflected from the surface, and is not affected by edge scattering as is the case with apertures with a non-zero thickness. Such scattering limits the maximum resolution of the device. Fourthly, the use of reflective grating elements allows the use of well known and accurate photographic or metallising techniques, for example metal on glass. The use of these techniques with apertures may result in loss of resolution or other problems from internal reflection, diffraction or degradation over time as the EMR has to travel through the glass between the metallised regions.
Finally, the use of reflective grating elements allow the use of intermeshed castellations which can provide a lost motion connection limiting the maximum angular deflection of the torsion bar, thereby eliminating the need for a separate torque limiting device and reducing the cost and complexity of the transducer.
The present invention consists in a torque transducer comprising a rotating shaft at least partially surrounded by a fixed housing, the axis of rotation of the shaft fixed with respect to the housing, the shaft comprising first and second substantially rigid torque input members which are connected by a torsionally compliant coupling, the coupling thereby enabling angular deflection of the first torque input member relative to the second torque input member as a function of the magnitude of the torque in the shaft, a first grating element attached to or integral with the first torque input member and a second grating element attached to or integral with the second torque input member, the first grating element comprising a first surface and the second grating element comprising a second surface, the transducer also comprising one or more electromagnetic radiation (EMR) sources and one or more arrays of EMR sensitive detectors, characterised in that each source irradiates one or both of the surfaces and each array receives incident EMR reflected from one or both of the surfaces, the one or more sources irradiating each surface and the one or more arrays receiving incident EMR reflected from this surface are all positioned in the same side of this surface and fixed with respect to the housing, both surfaces comprise alternating regions of high and low reflectivity, a pattern produced by incident EMR on each of the one or more arrays at any instant of time resulting from the alternating regions of low and high reflectivity on the one or both surfaces providing reflected EMR to this array regardless of the angular position of the shaft and irrespective of the relative angular deflection of the first and second torque input members, the output from the one or more arrays, resulting from the pattern or patterns on the one or more arrays at said any instant of time, is processed by a processor to derive the relative angular deflection of the first and second torque input members, and hence provide a measure of the magnitude of the torque in the shaft.
In some embodiments of the present invention a first array receives incident EMR reflected from a first surface and results in a first pattern, and a second array receives incident EMR reflected from a second surface and results in a second pattern. It is preferred that the processor receives inputs from the first and second arrays, and the processor comprises software or hardware electronic means to determine the relative displacement of the first and second patterns.
In other embodiments of the present invention the first and second surfaces are either mutually adjacent or contiguous, a single array receives incident EMR reflected from both first and second surfaces and results in a single pattern, the pattern comprises a is first subpattern produced by the incident EMR reflected from the first surface and a second subpattern produced by the incident EMR reflected from the second surface. It is preferred that the processor receives inputs from the single array, and the processor comprises software or hardware electronic means to determine the relative displacement of the first and second subpatterns. It is preferred that the single pattern is an interdigital pattern comprising the first subpattern interposed between the second subpattern.
It is preferred that at least one of first or second surfaces is substantially cylindrical with a central axis collinear with the axis of rotation of the shaft, and the array, which receives incident EMR reflected from the at least one surface, is positioned radially inside or outside the surface. It is preferred that the at least one substantially cylindrical surface is discontinuous due the respective grating element comprising radially protruding castellations around its periphery, the castellations are substantially axially aligned, the regions of high reflectivity correspond to the areas of maximum radius of the castellations with respect to the central axis of the cylindrical surface, and the regions of low reflectivity are angularly aligned with the discontinuous gap areas or lesser radius areas between the castellations. Also it is preferred that the grating element is manufactured from metal or plastic material and the areas of maximum radius are smoothly machined, moulded or sintered, or surface treated with paint or material deposition to impart high reflectivity, and the discontinuous gap areas or lesser radius areas are machined, moulded or sintered, or surface treated with paint or material deposition to impart low reflectivity.
Alternatively, in certain applications, it may be preferred that the at least one substantially cylindrical surface is substantially continuous due to the respective grating element comprising a substantially smooth cylinder, the inside or outside surface of the cylinder comprising the alternating regions of high and low reflectivity, and the regions are substantially axially aligned. Preferably the regions of high reflectivity are. metallised, shiny or light coloured and the regions of low reflectivity are substantially transparent, roughened or dark coloured.
Alternatively, in certain applications, it may be preferred that the at least one of first or second surfaces is substantially radially disposed with respect to the axis of rotation of the shaft, and the array, which receives incident EMR reflected from the at least one surface, is positioned axially on one side of the surface. Preferably, the at least one substantially radially disposed surface is discontinuous due to the respective grating element comprising axially protruding castellations around its periphery, the castellations are substantially radially disposed, the regions of high reflectivity correspond to the areas of maximum axial protrusion of the castellations, and the regions of low reflectivity are angularly aligned with the discontinuous gap areas or less axially protruding areas between the castellations. Also it is preferred that the grating element is manufactured from metal or plastic material, the areas of maximum axial protrusion are smoothly machined, moulded or sintered, or surface treated with paint or material deposition to impart high reflectivity, and the discontinuous gap areas or less axially protruding areas are machined, moulded or sintered, or surface treated with paint or material deposition to impart low reflectivity.
Alternatively, in certain applications, it may be preferred that the at least one substantially radially disposed surface is substantially continuous due to the respective grating element comprising a substantially smooth disc or planar ring, one side of the disc or planar ring comprising the alternating regions of high and low reflectivity, the regions are substantially radially disposed, the regions of high reflectivity are metallised, shiny or light coloured, and the regions of low reflectivity are substantially transparent, roughened or dark coloured.
Preferably the array comprises a one dimensional or a two dimensional array, a CCD, a VLSI vision chip or a lateral effect photodiode.
Preferably the pattern or patters is also processed by a processor to derive angular velocity and/or the relative angular position of at least one of the torque input members Preferably surface of at least one grating element includes areas or additional regions of high or low reflectivity whose resulting pattern is also processed to derive absolute angular position of the torque input member to which the at least one grating element is attached to or integral with.
Preferably the alternating regions of high and low reflectivity on the surface of the at least one grating element are arranged in the form of a succession of individual binary bar codes arranged such that the individual bar codes do not overlap. Alternatively the alternating regions of high and low reflectivity on the surface of the at least one grating element are arranged in the form of a succession of individual bar codes arranged such that the individual bar codes overlap. The resulting pattern on the respective array is processed to derive the absolute angular position of the torque input member to which the at least one grating element is attached to or integral with. It is preferred that a succession of binary bar codes are employed on both grating elements and the difference in the absolute angular position of the first and second torque input members is used to provide a measure of the magnitude of the torque in the shaft.
Preferably the first and second grating elements are adjacent and comprise radially extending intermeshing castellations, clearance being provided between the castellations and thereby providing a rotational lost motion connection between the first and second torque input members and hence limiting the maximum angular deflection of the torsionally compliant coupling.